Three subregions of the chicken MHC (B system) encode respectively for B-F (class I), B-G, and B-L (class II) antigens. Exchanges between the B-F and B-G regions have been identified by alloimmune antisera and are maintained along with the parental haplotypes from which they arose. Together they afford unique genetic material for mapping the chicken MHC and for a variety of studies which have the potential of providing information on the roles of the subregions of the MHC in development and function of the avian immune system. Comparison of the functions of the subregions with those of H-2 in the mouse and HLA in man should provide a better understanding of the evolutionary significance of the subregions of the vertebrate MHC. To this end, a series of 12 recombinants involving B-F and B-G are immediately available for further immunogenetic study, and matings can now be made to produce recombinants of special significance. A recombinant with a large portion of the B-G region duplicated as a result of unequal crossing over will be used extensively in attempts to stimulate further unequal exchange of subregional segments. Alloimmunizations between recombinants that are similar or appear to be serologically identical with currently available reagents will be carried out in an effort to detect more subtle recombinational differences. Immunizations between selectively matched recipients and donors will be made using erythrocytes to elicit antibodies for B-F or B-G alloantigens and lymphocytes for B-L antigens. Some of the recombinants possess regional segments from parental MHC haplotypes which have been shown to influence regression of Rous sarcoma virus-induced tumors. Others involve exchange of segments between haplotypes of proven resistance and susceptibility to Marek's disease, a malignant lymphoma; data from within family segregation under challenge has shown that resistance is due to a gene or genes in or closely linked to the B-F subregion. These recombinants will be used in collaborative studies of the subregions involved in resistance to tumors and in molecular characterization of functional products of the subregions.